Due to the recent improvement of the performance of physical computers, for example, a plurality of virtual computers (virtual machine: hereinafter, also referred to as VM as appropriate) can be operated on the physical computer.
In a known computer system, a plurality of users can each update a snapshot (Writable SnapShot, hereinafter, referred to as WSS as appropriate) that can be updated in a virtual environment (e.g., PTL 1). In PTL 1, the computer system includes a NAS OS (Network Attached Storage Operating System) having a WSS function, and the NAS OS provides a plurality of virtual NASs to each domain. The plurality of virtual NASs use the same original data, and each virtual NAS manages a differential corresponding to update data, whereby each domain can update the snapshot.